(1). Field of the Invention
This invention relates, in general, to silk-screen printing. More particularly, the invention relates to the silk-screens used in a multicolor half tone silk-screen printing process, such as the four-color silk-screen printing of the surface of a compact disc, and to the method for manufacture of those screens.
(2). Description of the Prior Art
Silk-screen printing, in general, involves the use of a screen, i.e., a woven mesh fabric, stretched over a frame and the design or text to be printed is provided on the screen in outline form, in the nature of a stencil. The design to be printed is provided in the silk-screen, in general, by coating the screen with a photosensitive emulsion, exposing the emulsion to obtain the desired image and then washing the unexposed areas to leave the screen with the image to be printed. The design or text is reproduced on a desired object, e.g., the surface of a compact disc, by having a squeegee force color, i.e., ink, through the mesh of the exposed areas of the screen. Thus, the image or text printed comprises a plurality of closely spaced dots of color.
Half-tone printing, contrary to full color printing, involves a shading or gradation of color. In such printing, the gradation of the tone of color is obtained by a system of closely spaced dots arranged in parallel lines. For example, in the four color printing of the surface of a compact disc, this involves the separate printing of a line of dots of cyan, black, magenta, and yellow of full color, in turn. The dots of different color being printed need be provided in proper linear registration in respect to one another to provide the desired color tone. Thus, a line of full color yellow dots may be printed on the surface of a compact disk followed by the printing of a line of dots of magenta. These lines of dots of different color must be printed at predetermined angles, i.e., the angles of color separation, and in proper registration with one another to provide the desired gradation of color and to prevent moire'. The lines of dots of color are provided at predetermined angles so that the lines do not cross one another. The angles of color separation depend to some extent upon the particular printing process involved, i.e., offset, silk-screen, etc. It is important that the three darkest colors involved, i.e., magenta, black, and cyan be at angles 30.degree. apart from one another, i.e., 15.degree., 45.degree., and 75.degree., respectively. The yellow color separation should desirably be at either 0.degree. or 90.degree.. An almost endless number of color tints can be obtained by combining the four basic colors. Nevertheless, as is known by those skilled in the art of silk-screen printing, these angles follow the so-called "North American" offset standard and differ somewhat from the "European" offset standard. These angles, at which lines of dots of color should be printed in halftone printing do not necessarily, however, provide the ideal solution for silk-screen printing. Many such printers have developed color separation angles with their color separators that will work with the mesh counts usually used in silk-screen printing processes.
Registration of the lines of printed dots to one another to provide a suitable halftone presents no particular problem in printing processes other than in silk-screen printing. For example, in an offset printing process, registration can be relatively easily accomplished. Nevertheless, in the case of silk-screen printing, the proper registration of the lines of dots of color being printed is of particular concern. Otherwise, a unique problem called "moire" occurs. Moire' can result for several reasons. One reason is that the lines of dots of ink being pushed through the mesh of the silk-screen are not uniformly spaced apart from one another. Some lines of dots are at the desired spacing, and others are not. Other lines of dots of color may be spaced closer or further apart than is the desired spacing. The nonuniform line spacing is, of course, due either to the nonuniform spacing of the warp and weft threads forming the woven screen or the fact that the mesh or openings in the screen being used are not properly aligned in a straight line. These problems are the result of the lack of uniform stretching of the woven fabric from which the screen is made. Where this occurs, the warp threads may be at a different degree of tension than the weft threads and some movement of next adjacent threads away or toward one another may occur in the screen during use.
Another reason for moire', and perhaps the major reason, is the fact that the line of dots of one or more of the color separations may not be provided at the desired angle relative to the mesh in the screen. Thus, for example, in the first color being printed, if the angle of the dots being printed is only slightly different from the angle of color separation, moire' will occur even though the other colors being printed corresponds to that of the color separations. Or, on the other hand, the first color printed may be at the correct angle, and one or more of the subsequent colors printed be at an improper angle.
Moire' manifests itself in a series of visual bands which present an unsuitable shimmering pattern or wave-like appearance in the gradation of color on the object being printed. Moire' is a particular problem in the four-color printing of objects, e.g., the surface of a compact disc. In this case, the color separation for each of the halftone screens must be at the proper angle, to avoid the moire' effect.
Heretofore, the manner of eliminating, or at least reducing, moire' in silk-screen printing has been most difficult. Changing the angle or size of the screen, i.e., the mesh count of the woven fabric used in the manufacture of the screen, usually solves the problem. Nevertheless, the manufacture of a silk-screen for halftone printing is a somewhat costly and labor intensive operation. Where moire' occurs, the making of a new set of screens with different color separation angles or a different mesh woven fabric, or both, merely compounds this expense to a printer.
The manufacture of a silk-screen for use in the printing of halftones involves a number of individual steps. The first step is to make color separations of the halftone image involved. This is done at a predetermined line count, preferably at a line count of 120 lines of dots of color per inch, as such gives some latitude in the choice of a woven fabric for use in making a silk-screen. The color separations may be made by either the printer or the screen manufacturer, generally by the printer. Where the color separations are made by the printer, the silk-screen manufacturer nevertheless confirms the halftone line count for each of the color separations, using a halftone calculator according to conventional techniques. In four color printing, this involves four different color separations; hence, the screen manufacturer need make four confirmations.
In general, the confirmation of line count involves the placement of each color separation or film positive, in turn, on a light table having a single point incandescent light, e.g., a 60 watt bulb, and taping the color separation in place, emulsion side up. This is accomplished after the crop marks provided on the film positive, at 3, 6, 9 and 12 o'clock are lined up, in usual manner, with those provided on the light table. The Calculator is then rotated on top of each of the color separations. The largest moire' effect visually observed indicates the line (or dot) count. This process is repeated for each of the colors involved.
Next, using a conventional Star Guide Orientator, the halftone angle for the lines of dots of color for each color separation is determined. This is done, in general, by rotating the Star Guide on top of the film positive until the moire' effect is again obvious. This indicates the angle of separation for that particular color. The same light source is used for the angle determination, as for the line count. In general, the preferred angles for color separation are cyan (75.degree.), black (45.degree.), magenta (15.degree.), and yellow (90.degree. or 0.degree.), these being the North American offset standards earlier disclosed.
Next, one determines the proper thread count for the halftone silk-screen printing to be done, i.e., the woven fabric to be used in the manufacture of the silk-screen whereby moire' will be eliminated, or at least reduced. This is done by reference to a conventional chart, disclosed more fully hereinafter, showing the relationship between halftone line count and the thread count of a woven fabric and moire'. In general, the higher the thread count, the least likely that moire' will result, even over a relatively large range of halftone line count. For example, at a thread count of 465 threads/inch, there is little possibility that moire' will result even at a halftone line count varying from 85 lines/inch to 133 lines/inch. A line count of 120 lines/inch is preferred, however, as the chart shows there to be little likelihood of moire' occurring at a mesh count of either 420 or 465 threads/inch. Thus, based upon the line count determined for the color separations and thread count relationship, one chooses that woven fabric having a thread count for manufacture of silk-screens least likely to result in moire'.
The next step is to determine the angle of mesh for each of the color separations at which moire' can be least detected by visual observation. This is done, in general, by laying a 90.degree. screen of the right mesh size over the taped positive on the light table. The screen is then rotated by hand until moire' can no longer be detected, or at least until moire' is localized and minimized. The rotation of the screen for moire' to be no longer detected may need be in either a clockwise or counterclockwise direction. A conventional protractor is then used to measure the angle that a moire' free screen varies from the starting position, and in which direction. This procedure is followed for each of the colors involved.
The mesh angle is sometimes confirmed by rotating the film positive rather than the screen, the film positive being located at the angle earlier determined. The 90.degree. screen is then set square over the positive. If moire' hasn't changed then this is the angle at which the woven fabric mesh is located relative to the screen frame in the fabric to be stretched to manufacture the silk-screens.
A suitable size woven fabric, square cut, and of the thread count previously determined, is then located in usual manner in stretching apparatus and stretched. This is accomplished by grabbing each of the opposed edges of the fabric with an elongated stretching bar. The warp threads terminating in for example, opposed end edges are stretched in opposite directions, and the weft or fill threads terminating in opposed side edges are stretched in opposite directions. The screen fabric should be stretched to a tension of at least about 16-20 newtons, an even higher tension being more desirable. The higher the tension, the less off contact is needed between the silk-screen and object being printed, e.g., a compact disc, to be printed. The higher the tension in a screen, the more it springs back after ink is pushed through the screen mesh, which helps to prevent distortion in the final printed image. Importantly, however, all the screens to be used in, e.g. the four color printing process, must be at a consistent tension.
The highest tension to which a woven fabric should be stretched is, of course, set by the fabric manufacturer. As is usual, a woven fabric following stretching, relaxes to a certain extent. This is, of course, taken into consideration when considering the tension to which a fabric should be stretched, the tension at any one time being determined in usual manner by conventional tension gauges. Certain mesh fabrics relax more than others. Nevertheless, after a period of use, the average relaxation period can be determined. A woven fabric in many cases is stretched to a 20% higher tension than necessary for the finished screen. Once the woven fabric is stretched to the extent desired, the silk-screen frames corresponding in number to the desired color separations are placed in contact with the bottom surface of the stretched fabric and the woven fabric stretched over the frames. Adhesive commonly used for this purpose is then applied to the top surface of the woven fabric, seeping downwardly through the mesh in the fabric to the frame. Prior to application of the adhesive, however, each frame is oriented on the stretched fabric at the desired angle, i.e., the angle at which it was previously determined, as above disclosed, that moire' could no longer be detected. This is accomplished, in general, by means of a conventional protractor placed along an edge of the frame. The stretched fabric is thus adhered to the silk-screen frame. This process is repeated for each of the colors involved in the color separation.
The screens are then each provided with the halftone image to be printed by that screen. This is done, in general, by first applying a photosensitive emulsion to the screen. A film positive e.g. of the color separation of cyan, is then placed against the emulsion and the emulsion is exposed by light. The unexposed areas of the emulsion are then washed out in conventional manner. This procedure is repeated for each of the color separations. The screens are then ready to be used in a silk-screen printing operation.
The determination of the angles for the mesh of the woven fabric relative to the screen frame to be used in the manufacture of a set of silk-screens for halftone printing, as earlier described, and rotating of the chosen woven fabric or film positives until it appears to the human eye that moire' is eliminated, or at least more localized, is largely a hit and miss procedure. The same is true with respect to the rotation of a screen frame to be adhered to the stretched woven fabric. As will be appreciated, these procedures depend to a large extent upon the eyes of the person aligning the film positives or woven fabric relative to one another, and the accuracy of the placement of the protractor. The carry over of that angle to the placement and orientation of screen frames on the stretched fabric using a protractor further compounds the problem.
Thus, it is highly desirable that an improved method for the providing of a set of silk-screens for use in the multicolor silk-screen printing of halftones, whereby moire' can be eliminated, or at least localized and minimized, be provided. It is also desired to provide apparatus for accomplishing the manufacture of silk screens that eliminates much of the guess work now involved in screen manufacture for printing halftones.